The present invention relates generally to power supplies and more specifically to isolated switching power supplies.
Efficient and ever smaller size power supplies are in high demand in almost all electronics devices in a wide range of applications. For example, smaller and more efficient power supplies are needed in telecommunication and embedded system applications, Power-over-Ethernet (POE) applications, microprocessors and chipsets requiring precise and robust voltage regulation, personal computers, cellular telephones, personal digital assistants (PDAs), etc.
Power supply architectures can be classified according to the structure of their power stage and associated properties. One such power supply is a flyback power supply. FIG. 1 shows a flyback power stage circuit 100 having a transformer consisting of three coils 104, 108, and 112. Coil 112 is the primary coil, coil 104 is the feedback coil, and coil 108 is the secondary coil. The secondary coil 108 is connected to a diode 116 and a capacitor 120 to rectify and smooth the output voltage signal. The output voltage Vout 120 is the voltage between the two endpoints of the capacitor 120. The input voltage Vin 122 is the fixed potential of one end of the primary coil 112 relative to the system ground. The feedback voltage 132 is the voltage of the node 134 relative to the system ground. It is typically proportionally related to the voltage across the feedback coil 104 through a resistive voltage divider.
A metal oxide semiconductor field effect transistor (MOSFET) 124 is typically connected to one endpoint of the primary coil 112. The MOSFET 124 switches on and off, thereby transferring electromagnetic energy from the primary coil 112 to the secondary coil 108. The switching MOSFET control waveform duty cycle sets the output voltage level. It also induces changes in the voltage between the two endpoints of the feedback coil 104 (and, thus, in the feedback voltage 132). When the MOSFET 124 switches on and off, the feedback voltage 132 changes. Therefore, the feedback voltage 132 is a pulsed waveform.
The output voltage Vout 120 can be measured indirectly via the feedback coil 104. Thus, the output voltage Vout 120 can be measured indirectly from the feedback voltage 132. At specific time intervals the feedback voltage 132 is proportional to the output voltage 120. The feedback voltage 132 is compared to a reference voltage level to compute an error signal used to modify the MOSFET switch control waveform and close a control loop that regulates the output voltage 120 at a wanted level.
The flyback circuit 100 uses analog components to indirectly measure the output voltage Vout 120. The analog components measure the feedback voltage Vout 132 during a specific time period. In particular, when the MOSFET 124 is turned “on”, the primary coil 112 obtains a fixed, non-zero voltage. When the MOSFET 124 is then turned “off”, the feedback coil 104 obtains a voltage (through induction) and a feedback pulse appears. Thus, the feedback voltage is measured during the time intervals that the MOSFET 124 is switched “off”. However, due to leakage inductance effects primarily which cause an initial voltage spike, it takes a finite time until the feedback voltage 132 accurately represents the output voltage 120. In order to extract the relevant output voltage information from the feedback voltage 132, a fixed delay is introduced between the MOSFET 124 switch-off command and the enabling of feedback-voltage-measuring circuitry. This fixed delay is set by a choice of resistors and capacitors.
Because resistors and capacitors are analog components, there are traditionally imperfections associated with each of them. These imperfections can result in imprecision with respect to their listed value. Thus, a resistor may have an actual value that is slightly different than its listed value. These imperfections will likely have an effect on the resulting time. Further, the values of resistors and/or capacitors may fluctuate with temperature changes. As a result, many attempts may be needed to determine which resistors and which capacitors to use in order to accurately measure the feedback voltage. Additionally once component values have been chosen to implement a certain fixed delay, these components would have to be replaced to implement another fixed delay. This can be required if the power supply is used under different load conditions.
Therefore, there remains a need to more accurately determine output voltage information from the feedback voltage of a flyback circuit.